IPT systems, and use of a pad including one or more windings that may comprise the primary or secondary windings for inductive power transfer, are introduced in our published international patent application WO 2008/140333, the contents of which are incorporated herein by reference. One particular application of IPT power transfer pads is electric vehicle charging. IPT power transfer pads are used both in the vehicle as a power “pick-up” device (i.e. the secondary side winding of the IPT system), and at a stationary location such as a garage floor as the “charging pad” (i.e. the primary side winding) from which power is sourced.
In the development of pick-ups for inductively charging electric vehicles a problem of some concern is the clearance available under the vehicle. With conventional pick-up circuits power in sufficient quantities can be provided at distances up to perhaps 100 mm at which time the coupling factor becomes so small that it becomes impractical.
It is generally conceded that the power required to charge a typical electric vehicle overnight is about 2.0 kW, so that in an overnight charging mode some 24 kWH can be transferred. With modern electric vehicles this is enough energy to travel more than 100 km and is ideal for small vehicles used for tasks such as dropping children at schools, running errands, short commutes and the like.
Inductively coupled chargers commonly use two power pads that are circular in shape and may have dimensions of 400 mm diameter by 25 mm thick as shown in FIG. 1. However, to use an inductive charger such as this the vehicle must be positioned relatively accurately over the charging pad—typically within 50 mm of perfect alignment—and the separation between the power pad on the vehicle and the power pad on the ground must be closely controlled. In principle inductive power transfer may be accomplished for vertical spacings between 0 mm and 100 mm but if the system is set up for 100 mm it will have quite a large reduction in power at 120 mm and will be inoperable below 50 mm. This state of affairs occurs because both the self inductance and the mutual inductance of the power pads vary widely as the distance between the pads changes. The self inductance and the mutual inductance as a function of the separation for two identical circular pads that have the construction of FIG. 1, are shown in FIG. 2. Thus at 100 mm the power pad receiver or pick-up may have a pick-up voltage of 100 V and a short circuit current of 5.0 A for a power rating of 500 W. If the IPT system electronics operates with a Q factor of 4, then 2 kW can be transferred to the battery though there are still difficulties to overcome in producing the power needed at the appropriate battery voltage.
The induced voltage in the pick-up pad (i.e. the vehicle mounted power pad) is very separation sensitive—corresponding to the variation in mutual inductance shown in FIG. 2—so that at 120 mm it is reduced by approximately 40% while at 50 mm it is increased by a factor of 2. A reduction in power means that the vehicle does not get fully charged in the usual time, but the more challenging situation is that at smaller separations the power transferred may be so high that the components of the circuit are overloaded. Also, as the separation is reduced the self inductance of the pick-up coil also changes so that the circuit operates off-frequency putting extra stress on the power supply. As the separation gets smaller still this stress on the power supply caused by the non-tuned pick-up on the primary side cannot be sustained and the system must be shut down. In practice it is feasible to operate with a separation between 40 and 100 mm but a larger range is too difficult.
A range of separation from 40 to 100 mm is quite small. If the vehicle has a relatively high ground clearance then either the power pad on the vehicle has to be lowered or the power pad on the ground has to be raised. Automatic systems for doing this compromise the reliability of the charging system. Alternatively the pad on the ground can be on a fixed but a raised platform but such a pad is a tripping hazard when a car is not being charged and this situation is generally to be avoided in a garage or other location involving vehicles and pedestrians.
The known power pad construction of FIG. 1 comprises an aluminium case 1 containing typically eight ferrite bars 2 and a coil 3. Current in the coil causes magnetic flux in the ferrite bars and this flux has flux lines that start on the ferrite bars and propagate to the other end of the bar in a path containing the coil that may be thought of as a semi-elliptical shape. The flux lines 4 for a single bar are shown in FIG. 3. The flux lines leave the ferrite in an upward direction and propagate to the other end of the bar, entering it at right angles. No flux goes out the back of the pad as the solid aluminium backing plate 1 prevents it. In the actual pad the eight bars give a flux pattern shown approximately in cross section in FIG. 4. A simulation of the actual flux pattern is shown in FIG. 4A.
From FIG. 4A it can be seen that at the highest point the flux lines are essentially horizontal. Therefore, to get the maximum separation possible between the primary pad and the secondary pad it would be advantageous to detect this horizontal flux. However, the horizontal flux is still relatively close to the pad (extending from the pad approximately one quarter of the diameter of the pad) and there is no horizontal flux at all at the very centre of the power pad. Thus at the very point where maximum flux density would be ideal—the centre—the actual usable horizontal flux component is zero.